Digestion of vegetable material by nitric acid



May 31, 1966 J. x. DESFORGES ETAL 3,253,977

DIGESTION OF VEGETABLE MATERIAL BY NITRIC ACID Original Filed Dec. 16, 1959 United States Patent() 3,253,977 DIGESTION F VEGETABLE MATERIAL BY NITRIC ACD Jean X. Desforges, Auburn, Mass., and Robert L. Somerville, Neshanic, NJ., assignors to Napex, Inc., Grand Junction, Colo., a corporation of Colorado Continuation of application Ser. No. 859,875, Dec. 16, 1959. This application Apr. 15, 1965, Ser. No. 451,093 Claims. (Cl. 162-81) This application is a con-tinuation of application Serial No. 859,875, filed December 16, 1959 and now abandoned.

This invention relates to digestion of cellulose fiber bearing vegetable material by nitric acid to solubilize lignins and render cellulosic components of the material.

Chemical pulping is carried out commercially by various processes including the sulfate process, the sulfite process and the soda process. These processes in general have the disadvantage that they are most suitable for production of pulp from specific raw materials such as soft woods. Thus, in general, the commonly used .processes are not suitable for production of pulp from hardwoods. The shotcoming of the commercially used processes has prompted workers in the art to search for a more generally applicable process, and these efforts have often resulted in proposals to employ nitric acid as the digestion reagent. Nitric acid is suitable for digesting cellulose containing vegetable material generally, but, so far as is known, none of the proposals to employ nitric acid have been commercially successful. One `reason for this failure is that the proposed processes are uneconomical. The principal object of the present invention is to provide a feasible nitric acid pulp production process.

It has been found that nitric acid can be used successfully by employment of particular conditions in the process. These conditions are concerned principally with the digestion, but conditions for other of the operations are also important and if suitable provision is made therefor, significant improvements in the overall process can be realized. Thus, operations other than the digestion for which special provision can advantageously be made are the conditioning of the digester eiuent liquor from the digestion t-o render it suitable for re-use in the process, the purging of impurities from the liquor, and the recovery of cellulosic values by extraction ofthe solubilized lignins. The invention makes suitable provision for the digestion and also for the other operations referred to, and thereby provides a more feasible pulp production process.

In the practice of the invention, the material flow can involve movement of the cellulosic material, in divided form as is common in pulping processes, through a digestion zone, then on to a washing operation, and then through an extraction operation. The liquor can be circulated between the digesti-on zone and a conditioning zone. Gas evolved in the digestion and conditioning zones can be utilized for production of nitric acid which can be returned to the liquor cycled between the digestion zone and the conditioning zone. Purging of non-volatile impurities from the liquor can be by withdrawing a portion of the liquor, purifying it and returning values recovered therefrom to the cycle. The various operations which are important to the successful practice of the invention are described in detail hereinafter.

The digestion reaction involves the action of nitric acid on the vegetable material and a corresponding reduction of nitrogen of the nitric acid. The action of the nitric acid may be oxidation or other degradation mechanism and may involve destruction of ether linkages by, for example, rupture -or hydrolysis. The reduction of nitric acid can be tolerated provided it is limited so that the nitrogen is not reduced to a valence state from which it cannot be conveniently oxidized back to nitric acid.

3,253,977 Patented May 31, 1966 reduction can 'be so limited, provided the nitrous ion con-4 tent of the liquor is maintained at a relatively low value.

According to the invention, the liquor can be treated prior to digestion to reduce the nitrous ion content thereof and the digestion is carried out in the manner that the nitrous ion content of the Iliquor is maintained at a low value. In consequence of these conditions, reduction of nitrogen to a valence state of less than 2'is minied.

Nitrous ion, as nitrous acid, is normally present in nitric acid aqueous solutions in the amount of about 25 p.p.m. of nitrogen as nitrous ion in 40 B. (870 grams per liter) nitric acid of commerce. Upon the occurrence of oxidation reactions involving the nitric acid, the nitrous ion concentration increases substantially. Thus, if 150 grams per liter acid (i.e. 150 grams of nitric acid per liter of solution) is used in oxidation reactions carried out at about 70 C., the nitrous ion concentration will reach an equilibrium value in excess of 50 ppm. nitrogen. In the digestion reaction of the invention, nitrous acid tends to form during the course of the digestion and this has the undesirable effect referred to above, namely it significantly increases nitric acid consumption by promoting its reduction to a state from which it cannot be conveniently transformed back to nitric acid. The nitric acid is re-used in the digestion and in the absence of treatment to limit the nitrous ion content of the liquor, the nitrous ion concentration would build up to in excess of 100 p.p.m., and likely substantially in excess of 100 ppm., e.g. in excess of 1000 ppm. Accordingly,lit is advantageous to reduce the nitrous ion concentration in the liquor before it is introduced into the digestion zone. Various means can be employed to reduce the nitrous ion concentrationof the liquor and a preferred means is described herreinafter. It has been found, however, that merely reducing the nitrous ion content of the liquor employed for the digestion, is of limited effectiveness unless provision is made to maintain the nitrous ion content at a low level d uring the digestion.

Formation of nitrous acid during digestion can be limited by circulating the liquor through the vegetable material at a high rate, and also by passing the vegetable material and nitric 'acid liquor countercurrently through the digesting zone. By employing a high nitric acid throughput rate, contacting time is limited so that the tendency toward increased nitrous ion concentration during digestion, is anticipated. The explanation for theV direction, which are signicant to the eiciency of the digestion are the temperature and concentration of nitric acid. The amount of liquor in the system is also signifi- Cant and it is desirable to maintain the volume of liquor at a low value since low liquor volume is compatible with low nitrous ion concentration and facilitates maintaining the nitrous ion concentration at a low level.

More particularly, the digestion conditions which have` been found to provide significantly improved results are an initial nitric acid concentration (i.e. nitric acid concentraton of the liquor upon introduction into the digestion zone) of about -210 grams per liter and preferably about-l25-l75 grams per liter, with an optimum value of about 140-150` grams per liter; an initial nitrous ion concentration of less than the equilibrium value, for example,A less than about 30 ppm. nitrogen as nitrous ion, preferably less than about 10 p.p.m. and optimally less than about 3 ppm., for example, about 0.5 p.p.m., while maintaining the nitrous ion concentration during the digestion at less than the equilibrium value, for example, less th-an about 50 p.p.m., preferably less than about 30 p.p.m. and optimally less than about 3-5 ppm.; a relative liow rate on a weight basis of liquor to vegetable material of at least 50 to l and preferably in excess of 200 to 1, for example, 400 to l, and further a relative liow rate such that contacting time of chips and liquor is less than about 15 minutes, preferably less than about 8 minutes; and a temperature in the range of about 65- 75 C., preferably about 6G-71 C.

The digestion is particularly sensitive to the temperature and within the range of 65-75 C., the digestion is sensitive. The optimum temperature is generally about 68.5 C. Temperature variation over the digestion zone is preferably small. Advantageously the variation does not exceed 1 C. and preferably is not more than about 03 C. The high liquor circulation rate which is desirable because it facil-itates maintaining a low nitrous ion concentraton in the liquor as is set forth above, makes possible attainment of a limited temperature gradient through the digestion zone.

The residence time of chicks in the digestion zone depends on the nature of the material and chip size. Generally, it will vary from 5 minutes for bagasse to 21/2 hours for certain resinous soft woods. Hard woods will usually be retained for l to l-1/2 hours. Dead Engleman spruce, a pulp source for which the process of the invention is well suited, will usually be retained for a period of about l11A-1% hours.

The digestion is not particularly sensitive to the nitric acid concentration of the liquor. In the lower and higher portions of the range of 90-2110 grams per liter, efficiency is lower. Values outside of the range of 90-210 grams per liter could be used but in general such operation will be undesirable. We have found that best results are obtained when the nitric acid concentration 'is within the preferred ranges mentioned above. Further, it is preferred that the nitric acid concentration gradient through the digestion zone be relatively low. Here again the high iiow rate employed results in improvement in the digestion since the high flow rate limits concentration gradients in the digestion zone.

With respect to nitrous ion content of the liquor during the digestion, where countercurrent iiow of liquor and chips is employed, the time interval during which the chips and liquid are in contact adjacent the liquor draw olf from the digestion zone is very short, and, therefore, a high nitrous ion content adjacent the liquor draw off is not particularly significant. The liquor is quickly swept from the contact with the chips in this portion of the digestion zone and so higher nitrite concentrations, even above those mentioned above as suitable, can be tolerated adjacent the draw oif. Thus recycled nitric acid and, or make-up acid, even if the nitrous ion concentration thereof is high, could be introduced into the digestion zone adjacent the liquor draw off. It is suiiicient for the purposes of the invention Aif nitrous ion content of the liquor is in accordance with the above-mentioned suitable values substantially throughout the contacting in the digestion zone.

Conditioning of the digester eliiuent liquor from the digestion zone is carried out in a manner effective to reduce nitrous ion content to the extent of nitrous ion increase during digestion. Advantagously, this can be done by exposing the spent liquor to a reduced pressure, whereupon the probable decomposition reaction is as follows:

A preferred manner of conditioning is by way of stripping or vacuum distillation of the digester eiuent liquor. Conditioning in this manner permits controlling the temperatureof the liquor in the digestion zone b-y selection of a suitable vacuum for the stripping.l If desired, the treatment by stripping can be supplemented by adding urea to the stripper efliuent in order to reduce nitrous ion concentration to below that which would be practical or possible by stripping alone.

Purging of non-volatile impurities from the liquor cycled between the digestion zone and low pressure or conditioning zone, is necessary to prevent a build up of materials, for the most part organic compounds, which would have a detrimental effect on the digestion, one such effect being that the impurities seem to increase consumption of nitric acid.

The purg-ing can be conveniently worked into the conditioning procedure. Thus, a portion of the liquor cycled between the digestion zone and the low pressure zone can be Withdrawn from the cycle and circulated through a reboiler operatively associated with a stripping unit used for the liquor conditioning. The boil-up or vapor from the reboiler can then be employed to strip the digester effluent liquor in the stripping unit, and residue of the boiling, which is a concentrate of the cycle liquor, can be treated for recovery therefrom of nitric acid values. The recovered nitric acid values can be introduced into the liquor cycle. Depending on the nitrous acid content, the recovered nitric acid values can be introduced into the cycle liquor going to the digester from the stripper.

Purification of liquor withdrawn from the cycle as a concentrate obtained as described above or otherwise, is preferably by dialysis. In the dialysis, the liquor can be passed countercurrent to water, the water and liquor being separated by a permeable membrane, such as for example a polyvinylchloride membrane. Nitric acid diffuses through the membrane into the water to form the diifusate liquid and this diffusate can then be introduced into the liquor cycled between the digestion zone and regeneration zone. If the nitrous ion concentration of the diifusate is low, it can be introduced into the liquor passing from the conditioning zone to the digestion zone.

Alternative purification procedures to the dialysis are ion exchange, electro-dialysis, adsorption by activated carbon or silica gel, and molecular sieves. Spray drying can be employed, but it is not as economical as the dialysis.

Various methods can be employed for extraction of lignin from the digested vegetable material. A preferred extraction method is to contact the digested material, after suitable washing to remove liquor retained thereby, with a solvent liquor in the form of either hot water or an aqueous alkali carbonate solution, then separating the vegetable material and solvent liquor, and thereafter contacting the vegetable material with another solvent liquor in the form of an aqueous alkali hydroxide solution. The suitability of hot water for extraction is surprising and has not been known heretofore. Suitable water temperatures are up to about C. and preferably 80-95 C. Higher temperatures, e.g. about C., seem to result in undesirable reactions.

Various modifications of the extraction procedure as described above can be employed. Thus, the digested vegetable material can be passed serially through a hot water extraction step, an alkali carbonate step and finally an alkali hydroxide extraction. Alternatively, as set forth above either the hot water or alkali carbonate extraction alone can precede the alkali hydroxide extraction. A particularly effective extraction procedure to produce a high quality chemical pulp is to employ an alkali carbonate step and then an alkali hydroxide step, and to use as the alkali carbonate solution for the first mentioned step, the effluent liquor from the alkali hydroxide step, which has the requisite carbonate content as a result of the ontacting thereof with the digested vegetable materia The invention will be further described with reference to the accompanying drawing wherein a schematic flow diagram for an embodiment thereof is set forth. The process depicted in the drawing is merely representative of the invention and is not intended to convey the limits thereof. Various alternatives and modifications will occur to those skilled in the art.

Referring to the drawing, vegetable material in the form of chips or Hakes is conveyedthrough line 11 to screw conveyor 12, where it is wetted by steam introduced into the conveyor 12 through line 13. Condensate drains from the conveyor through line 14, and the conveyor feeds the wetted chips to a vertical screw conveyor 14 which 4moves the chips to another screw conveyor 1S. The screw conveyor 15 is ooded with wash liquor from the washing step later to be described. This last mentioned conveyor delivers the chips to the digester 16.

The effect of steaming and then immersing the chips in the relatively cool wash water is to thoroughly deaerate the chips. This treatment presupposes the use of dry chips and if the chips are green it may not be required, and the green chips can be fed as such to the digester 16. Instead of employing wash water to wet the chips, plant water can be used. If desired, dry chips can be introduced into the digester as such and without the deaeration pre-treatment.

The digester 16 is a drag flight conveyor arranged for operation at atmospheric pressure and to provide a contacting or digestion zone through which digestion liquor and chips can be moved countercurrently. The digester includes a conveyor element 17 which travels in the direction indicated by the arrows thereon and .advances chips down one side and up the other side of the conveyor to chip draw off 18. Digestion liquor having reduced nitrous ion content enters the digester 16 from line 19 and passes countercurrent to the chips through the conveyor to outlet line 20 whereby it is withdrawn from the digestion zone as digester effluent liquor. A weir 21 is .provided between the body of liquor in the digestion zone and the outlet line 20 since the outlet line 20 communi- Cates with the low pressure maintained in equipment shortly to be described. Line 2) can be provided with a control valve (not shown) to control flow therethrough.

The digester effluent liquor is directed by line 20 to vacuum stripper 22, wherein a low pressure or stripping zone is maintained and the digester effluent liquor is conditioned for re-use in the digestion zone. The vacuum stripper includes several theoretical plates and permits treating the liquor so as to expel nitrous acidtherefrom. The overflow from the bottom tray of the vacuum stripper enters line 19 which includes barometric leg 29 and pump 30, and connects the vacuum stripper with digester 16.

As thus far described, the process is operated in the manner that chips are continuously passed through the digester 16, and liquor is cycled between the digester 16, where it contacts the chips, and the vacuum stripper 22 where it is conditioned for re-use in the digester. The temperature of the liquor in the digestion zone is maintained at a level effective to promote digestion and during the digestion, nitrous acid tends to form. According to the invention, the liquor is circulated through the digester and vacuum stripper at a high rate. As discussed hereinbefore, by employing this condition contact time of the liquor and chips is limited and hence formation of nitrous acid is limited. Another advantage of this condition is that concentration gradients through the digestion zone are low and hence substantially the preferred concentrations can be maintained throughout the digestion zone. A still further advantage of a high circulation rate is that temperature control is thereby facilitated. This last mentioned advantage is discussed in more detail below.

According to the invention, a portion of the conditioned liquor passing through line 19, which connects the vacuum stripper and digester, is withdrawn via line 31, and is directed thereby to the reboiler system 32 of the vacuum stripper 22. The liquor fed to the reboiler system via line 31 is the only liquor present in the reboiler system, since all of the overflow from the bottom plate of the vacuum stripper22 is introduced into line 19. The reboiler system 32 includes reboiler 33, circulating line 34 and pump 35. The reboiler is provided with steam inlet 36 and condensate outlet 37. Boil-up from the reboiler is contacted with liquor being conditioned on the trays of the vacuum stripper 22 and provides the heat for the stripping and which in turn results in reduction of the nitrous ion content of the liquor. Residue from the reboiler is continuously withdrawn from the reboiler system through line 38, and the withdrawn residue is treated in a manner shortly to be described to recover nitric acid values therefrom.

In operation of the reboiler system, about 0.5-2% of the overflow from the lower plate of the vacuum stripper 22 can be passed via line 31 to the reboiler system, and a volume of about ten times the 0.5-2% draw off through line 31 can be circulated through the reboiler. The concentration in the reboiler can be from about ISO-350 grams per liter nitric acid, and the withdrawal through line 38 for purification can be about 0.5-2% of the liquor circulated in the reboiler. Additionally, about 1-2% of the amount circulated in the reboiler can be withdrawn through line 38 and passed back to the liquor cycle via line 39, whereby maintenance of. the plant water balance is facilitated.

Overhead vapor from the vacuum stripper 22 passes through vapor line 42 to surface condenser 43 which is provided with cooling water inlet 44 and outlet 4S. A portion of the condensate which collects in leg 46 enters line 41 which directs it to the top tray of the vacuum stripper 22. This provides a reux to remove nitric acid from the rising vapors. The balance of the condensate enters line 47 which directs it to condensate stripper 48. The condensate stripper is provided with steam coil 49, and by means of this coil, s'ufiicient heat can be provided to strip the condensate of dissolved gases including nitric oxide, and also to decompose any nitrous acid in the condensate. The gases expelled in the condensate stripper are conducted by line 50 to the surfacel condenser 43. The condensate leaves the condensate stripper by way of line 51, which includes a cooler 52, and directs the condensate to water storage tank 53. This condensate is of a purity substantially equivalent to distilled water.

The non-condensable gases from the vacuum stripper 22 include nitric oxide and the gases can be treated so that the nitric oxide is recovered as nitric acid. The gases leave the surface condenser 43 by way of line 54, which includes pump 55, and directs the gases to nitric acid recovery plant 57. The pump 55 can be the liquid seal type employing either water or nitric acid as the liquid seal, and the pump serves to provide a suitable vacuum in the vacuum stripper 22.

The nitric acid recovery plant 57 receives the noncondensables from the vacuum stripper 22, as is described above, and additionally the digester 16 is vented by line 5 8 to the recovery plant. The recovery plant includes a nitric oxide oxidation unit and suitable absorption equipment so as to permit recovery of nitric oxide as nitric acid. This equipment is not indicated since it is well known in the art. The nitric acid produced in the recovery plant 57 is returned to the process via line 61. Make-up nitric acid enters the plant through line 62 which introduces the make-up acid into line 61. If desired, make-up nitric acid can be produced in recovery plant 57 by burning ammonia and introducing the resulting nitric oxide into the oxidation unit of the plant 57.

A feature of the process of the invention is operation of the vaccum stripper so that tempera-ture of the liquor employed for the digestion isl controlled by selection of the absolute pressure maintained in the vacuum stripper. The temperature in the digestion zone should be accurately controlled, eg. the temperature is desirably 68.5 C. J 0.3 C. throughout the digestion zone. This condition can be' maintained by circulating the liquor at a high rate through the digestion zone and the vacuum stripper and by selecting as the pressure for the Vacuum lstripper the boiling pressure of the liquor at the selected temperature. For operation 4of the digestion at 68.5 C., the corresponding pressure for the vacuum evaporator would be an absolute pressure of a little less than 214 mm. of mercury.

Purging of non-volatile from the cycle liquor circulated between the digester 16 and vacuum stripper 22 is effected by recovering nitric acid values from reboiler residue withdrawn from the reboiler system 32 by lway of line 38. Line 38 includes the barometric leg 63 and cooler 64 and conducts the residue to a dialysis system 65, which includes first stage dialyzer 66 and second stage dialyzer 67. The residue passes through first stage dialyzer 66 as the dialyzer and leaves this unit through line 68 which directs it to evaporator 69. Evaporator 69 is provided with steam inlet 70 and condensate outlet 71, and serves to concentrate the dialyzate effluent of the first stage dialyzer to about the nitric acid concentration of the residue fed to the first stage dialyzer. The dialyzer as liquor effluent from evaporator 69 passes through 72, which includes a cooler 73, to the second stage dialyzer 67. The dialyzate eiuent from the second stage dialyzer 67 leaves the dialyzer system by way of line 74. This residue eliluent from the second stage dialyzer contains the organic impurities and can be treated for recovery of these impurities as by-products. The organic impurities are probably composed for the most part of degraded lignins, wood sugars, hemi-celluloses, etc., which are picked up in'the digestion zone. The liquid employed in the dialyzers to receive the nitric acid by exosmosis is Water from supply tank 53. The water enters the dialyzers via lines 75 and 76. Nitric acid values of the residue are transferred to the Water and the nitric acid rich liquid leaves the dialyzer units as the diifusate via lines 77 and 78 which conduct the diffusates to line 79, which, in turn, conducts them to the cycle between the digester and vacuum stripper.

In operation of the dialysis system, the residue introduced into the system can be about Z50-350 grams per liter nitric acid; and the efficiency of each dialysis operation can be about 80% for an overall nitric acid recovery of about 95% or better.

The representation in the drawing of the extraction system is set forth so as to indicate alternative procedures.

Prior to the extraction, the chips are washed to remove therefrom retained digestion liquor. Chips are conveyed by the chip draw off 18 to washer 83, which is a drag flight conveyor similar in construction to the digester 16. In the washer, the chips are moved -countercurrently to wash water introduced into the washer by line 84, which connects with water supply line 85. The wash water leaves the washer via line 86 which includes pump 87 and directs the wash water to screwconveyor wherein the wash water is admixed with the chips being fed to the digester 16, as is described hereinbefore. The nitric acid concentration in the expended wash water can be about 50-130 grams per liter nitric acid. Washing time depends on the material being treated. It can be, for example, about 1 hour. The chips substantially free of nitric acid leave the washer via line 88 and pass on to the extraction system.

The extraction system includes hot water extractor 90, carbonate extractor 91, and caustic extractor 92. Each of the extractors is a drag flight conveyor generally similar in construction to the digester 16. The flow of chips and liquor within each extractor can be concurrent, coun- -tercurrent or mixed flow.

The chips enter hot water extractor fromline 88 and leave this extractor vvia line 93 which conveys them on to carbonate extractor 91. The chips leave carbonate extractor 91 by way of line 94 which conveys them to the caustic extractor 92, and they leave the caustic extractor 92 via line 95. The chips in line 95 constitute the rendered cellulose and upon gentle mechanical action these chips will separate into individual fibers.

Hot water `for the hot water extraction is obtained from line 96 which connects with' water supply line 85 and includes heater 97. If desired and to effect process economy, the warm Water effluent from the condenser 45 of the vacuum stripper 22 can be used as the hot water or as part of the hot water for the hot water extraction. The liquor effluent leaves the hot water extractor via line 98. This liquor contains ligneous materials which can be recovered as by-products.

The carbonate liquor employed in the carbonate extractor 91 is the liquor effluent from the caustic extraction carried out in caustic extractor 92. Solvent liquor in the form of sodium hydroxide aqueous solution is introduced into the caustic extractor 92 by way of line 100, and the spent liquor leaves this extractor as a black liquor high in carbonate content by Way of line 101 which conveys the black liquor to carbonate extractor 91. The liquor from the carbonate extractor 91 leaves this unit as a brown liquor by Way of line 102. This brown liquor contains lignins which can be worked up as by-products.

As will be apparent from the drawing, the extraction system can be operated so that the caustic extraction and only one of the other extractions is employed. By-pass lines 103 and 104 make such operation possible.

A preferred manner of extraction is to employ only the carbonate and caustic extraction. By operating in this manner a caustic consumption of as little as about 5 pounds of 100% sodium hydroxide per 100 pounds of air dried pulp product, can be realized. For such operation the chips would be passed via by-pass line 103 directly from the washer 83 to the carbonate extractor 91. The Iextraction system would then involve a first stage extracting zone maintained in the carbonate extractor 91 and a second stage extraction zone maintained in the caustic extractor 92. In the first stage or carbonate extraction, the digested material or chips from the washer 83 and solvent liquor or carbonate solution from the second stage are admixed to extract solubilized material; and in the second stage partially extracted material from the first stage and the fresh solvent liquor or caustic solution are admixed. The temperature for each the first and second stages can be -105 C. and the contacting time about 30-90 minutes, preferably about 1 hour. The pH of the fresh solvent liquor to the second stage can be at least about 11, and the pH of the solvent liquor passed from the second stage to the first stage can be about 7-8, preferably about 7, and the pH for the solvent liquor after passage through the first stage can be about 4-5, preferably 4.5. In this operation, the flow of chips and solvent liquor through the two stages is countercurrent, but the flow Within each stage can be concurrent, counter- -current or mixed flow.

The invention is further illustrated in the following example, which is for practice of the method of the invention in a plant as is depicted'in the accompanying flow diagram, The reference characters in the example refer to the flow diagram.

Example Digestion-Dead Engleman spruce is chipped to a size of 5A" x 1" x Ms and the chips are fed continuously at a rate of 20 pounds per hour to a first screw conveyor 12 where they are soaked with steam. Residence time in the The first screw conveyor delivers `the `chips to a second screw conveyor 15 which is flooded with wash liquor obtained from a washing step later to be described. Residence time in second screw conveyor is minutes. The second screw conveyor 15 conducts the chips and wash water to digester i 16. Digestion liquor is introduced into the digester by way of line 19 and at a rate of 15.6 gpm. It passes countercurrently through the chips. The digestion con- Stripping-The digester eflluent liquor is subjected to l vacuum stripping in a 6 theoretical tray, 8" diameter stripper 22 equipped with a reboiler 33. The digester eluent liquor is fed to the third tray from the top and withdrawn from the bottom tray. The pressure is 205 mm. of mercury absolute. The throughput of the digestion liquor is 15.75 g.p.m. The liquor temperature is 68.5 C. in and 68.3 C. out. The nitrous ion content of the l-iquor in is 30 ppm., and of the liquor out is 1 ppm. Overhead vapors from the stripper are passed through a water cooled condenser and condensate is fed to the top tray of the stripper.

' A 12.5 gallons per hour portion of the stripper eluent 4or conditioned liquor is directed to the reboiler system 32 of the stripper 22. Heat for the reboiler is provided by introducing l p.s.i.g. saturated steam at the rate of 56 Ipounds .per hour.' Liquor withdrawal from the reboiler system is 6 gallons per hour of 312 gram per liter nitric acid liquor. Of this 6 gallons per hour,v 4.5 gallons per hourV is returned to the liquor withdrawn from the stripper 22 and goes to the digester 16, and 1.5 gallons per hour -is subjected to dialysis.

Dialysis- The dialysis is carried out in a Graver Water Conditioning Company Lab Model HI-SEP dialyzer having a polyvinylchloride membrane and a membrane area of 3.7 sq. ft. Water in the amount of 1.65 gallons per hour, and liquor in the amount of 1.5 gallons per hour are passed countercurrently through the dialyzer. The liquor and water inlet temperatures are each 40 C. The nitric acid concentration in the diifusate and dialysate, respectively, is 200 and 90 grams per liter. The nonvolatile impurity content of the feed to the dialyzer is 12% by weight, in the dilfusate 3% by weight and in the -dialysate 9% by weight. The dialyzate can be concentrated and subjected to another dialysis treatment to effect recovery of additional nitric acid. In' this example, the ditfusate is returned to the cycle liquor.

Washing-From the digester 16, the chips are conveyed to Washer 83. Water at the rate of 6.5 gallons per hour is passed through the washing zone countercurrently to the chips and the efuent liquor is 5 gallons per hour of 80 grams per liter nitric acid solution. The

remaining 1.5 gallons per hour is retained by the chips.

The washer effluent liquor is directed to the screw conveyor 15 where it is mixed with fresh chips.

Extract0n.-A sample `of the chips from washer 83 is placed in batch with 7 times their weight of aqueous sodium carbonate solution of pH 7 (.025 N) for 60 minutes at 100 C. At the conclusion of this carbonate extraction, the pH is 4.5. The chips are then separated from the carbonate solution and placed in batch with 7 times their Weight of aqueous sodium hydroxide solution of pH 12 for 60 minutes at 100 C. -At the conclusion of this alkali extraction the pH of the solution is 7.0. The chips upon separation from the alkali solution were kdefibered by gentle mechanical agitation and then screened, thickened, and ltered to o'btain a pulp mat.

The nitric acid consumption for this example is about 15 pounds of 100% nitric acid'per 100 pounds of air dry cellulose pulp product, and pulp production is about 50-55% of the dryweight -of the wood digested.

In the specification and in the appended claims, nitrous ion or nitrous acid or nitrite concentration in parts per million (ppm.) is parts by weight of nitrogen per million parts by weight of material, which nitrogen is present as the nitrous ion or radical, combined or uncombined. The concentration in ppm. is ydetermined by c olorimetric analysis employing the known a-naphthylamine, sulfanilic acid test, and aqueous sodium nitrite solutions as standard solutions, and the liquor to be tested as such.

Whereas particular conditions have been set forth herein for the process of the invention, and for the Engleman spruce wood species, the optimum conditions for any given application of the process will depend to some degree on factors peculiar to the particular application. Accordingly, experimentation and consideration of conditions peculiar to-the applicationv as well as consideration of the process as such can be resorted to in order to determine the most suitable manner for carrying out the method in a given environment.

Various modications of the method of the invention can be resorted to for attaining particular objectives. Thus, in order to make a high`a-cellulose,'some sulfuric acid could be used in the digestion.

As indicated hereinbefore, an advantage of the method of the invention is the general applicability thereof to cellulose ber bearing vegetable materials. It can be used, for example, for producing pulp from hard woods, soft woods, and bagasse. It may also be useful to produce pulp and linen from ax.

What is claimed is:

1. A process for digestion of cellulose ber bearing vegetable material to solubilize lignins thereof and render cellulosic components thereof, which comprises contacting the material, in a digestion zone with a liquor containing nitric acid at a nitric acid concentration, and at a temperature, and for a time suicient to solubilize ligneous components by reaction of nitric acid and the ligneous components in which nitric acid is reduced and a reduction product containing nitrogen values is formed therefrom, maintaining the nitrous ion content of the liquor at less than about 50 p.p.m. nitrogen as nitrous ion substantially throughout said contracting, and collecting said reduction product and converting it to nitric acid.

2. Process according to claim 1 and withdrawing liquor from said digestion zone and treating it in a treating zone to reduce nitrous ion concentration thereof, and returning the treated liquor to the digestion zone for contacting as aforesaid and cycling liquor between the digestion zone and the treating zone to maintain a substantially constant maximum concentration of nitrous ion in the digestion zone.

3. Process according to claim 2, wherein the liquor is subjected to a reduced pressure in the treating zone to reduce the nitrous ion content thereof.

4. Process according to claim 1 and withdrawing liquor from said digestion zone and treating it in a treating zone to reduce nitrous ion concentration thereof, and returning the treated liquor to the digestion zone for contacting as aforesaid and cycling liquor between the digestion zone and the treating zone, withdrawing a portion of the liquor cycled between the digestion zone and the treating zone and subjecting the withdrawn portion to dialysis to transfer nitric acid values thereof to a diifusate liquid and using the nitric acid rich diffusate in said contacting.

5. Process according to claim 1, wherein the nitric acid produced from said reduction product is utilized in said contacting.

6. In a process for digestion of cellulose fiber bearing vegetable material to solubilize lignins thereof and render cellulosic components thereof, which comprises contacting the material with an aqueous liquor containing nitric acid at a nitric acid concentration, and at a temperature, and fOr a time suicient to `solubilize ligneous components, the improvement which comprises prior to `said contacting treating the liquor to reduce the nitrous ion content thereof to less than about 30 p.p.m. nitrogen as nitrous ion and effecting said contacting by passing the liquor with resulting low nitrous ion content through a body of the vegetable material, the `flow rate of liquor through the vegetable material being at a rate to limit contacting time so that nitrous ion formation during said contacting is limited to maintain the nitrous ion content of the liquor less than.

about 50 p.p.m. nitrogen as nitrous ion substantially throughout said contacting.

7. The process according to claim 6, wherein said contacting is carried out in a contacting zone and the vegetable material and liquor are passed countercurrently through said zone during the contacting, the relative How rates of liquor and vegetable material on a weight basis being at least 50 parts of liquor to 1 part of vegetable material.

8. Avprocess yfor digestion of cellulose ber bearing vegetable material employing nitric acid to solubilize lignins of the material and render cellulosic components thereof `which comprises exposing to a reduced pressure a liquor containing nitric acid in a low pressure zone to decompose nitrous ion present therein and thereby provide for digestion a nitric acid liquor containing less than about 30 p.p.m. nitrogen as nitrous ion, providing Ivegetable material in a digestion zone, withdrawing digestion liquor from the low pressure zone and passing it through the digestion zone and thereby contacting the vegetable material with the digestion liquor, maintaining the temperature in the digestion zone at a level to promote digestion of the material, whereby, during the digestion the nitrous ion content of the liquor tends to increase, withdrawing liquor from the digestion zone as digester eluent liquor to limit the nitrous ion content substantially throughout the digestion zone to less than about p.p.m. nitrogen as nitrous ion, returning the digester effluent liquor to the low pressure zone and therein treating it to reduce nitrous ion content thereof as aforesaid and thereby condition it for re-use in the digestion zone, and cycling said liquor between the low pressure zone and digestion zone, whereby to eifect said digestion of the vegetable material.

9. The process of claim 8, wherein the nitric acid concentration of the digestion liquor upon introduction thereof into the digestion zone is 90-210* grams per liter, and the temperature in the digestion zone is in the range of about -75 C.

10. The process of claim 9, wherein the conditioning in the low pressure zone is by vacuum stripping, the vacuum stripping being carried out at a temperature and pressure to provide the digestion liquor at a temperature substantially equal to the temperature employed in the digestion zone.

References Cited by the Examiner UNITED STATES PATENTS 1,840,198 1/1932 Heimann 162-81 1,923,292 8/1933 Bassett 162-81 1/1953 Atkinson 162-96 

1. A PROCESS FOR DIGESTION OF CELLULOSE FIBER BEARING VEGETABLE MATERIAL TO SOLUBILIZE LIGNINS THEREOF AND RENDER CELLULOSIC COMPONENTS THEREOF, WHICH COMPRISES CONTACTING THE MATERIAL IN A DIGESTION ZONE WITH A LIQUOR CONTAINING NITRIC ACID AT A NITRIC ACID CONCENTRATION, AND AT A TEMPERATURE, AND FOR A TIME SUFFICIENT TO SOLUBILIZE LIGNEOUS COMPONENTS BY REACTON OF NITRIC ACID AND THE LIGNEOUS COMPONENTS IN WHICH NITRIC ACID IS REDUCED AND A REDUCTION PRODUCT CONTAINING NITROGEN VALUES IS FORMED THEREFROM, MAINTAINING THE NITROUS ION CONTENT OF THE LIQUOR AT LESS THAN ABOUT 50 P.P.M. NITROGEN AS NITROUS ION SUBSTANTIALLY THROUGHOUT SAID CONTRACTING, AND COLLECTING SAID REDUCTION PRODUCT AND CONVERTING IT TO NITRIC ACID. 